1. Field of the Invention
The present invention relates to a frequency detection circuit which detects whether or not the frequency of a clock signal is lower than a predetermined frequency. This circuit outputs an error detection signal when the clock signal frequency is lower than the predetermined frequency. The present invention further relates to a data processing apparatus provided with such frequency detection circuit.
2. Description of the Prior Art
A data processing apparatus such as a microcomputer operates in synchronism with a clock signal, and this may malfunction when the frequency of the clock greatly deviates. Thus, such a frequency detection circuit as detecting whether or not the frequency of the clock is lower than a predetermined value is provided. The detection output is used to reset the operation of a CPU. The circuit having such function is disclosed in, for example, a Japanese laid-open Patent Publication 2002-55130.
The frequency detection circuit described in the publication, includes a capacitor C and a resistor R. The capacitor C is subjected to charging and discharging every time when the level of a clock changes. An error detection signal is outputted when the charge or discharge is not completed within one cycle of the clock.
However, the conventional frequency detection circuit uses the capacitor C and the resistor R, so that there is a problem that detection characteristics vary in accordance to a power supply voltage, an ambient temperature, variations in manufacture, etc. It is thus difficult to accurately determine whether or not a clock frequency is lower than a predetermined value.
Also, characteristics of analog elements constructing the data processing apparatus described above vary in accordance with use conditions such as an ambient temperature after shipment, so that there was a problem that detection characteristics of the frequency detection circuit vary.
Particularly, in the conventional data processing apparatus described above, there was a problem that when a frequency of a clock is lower than a predetermined frequency (that is, the predetermined frequency in which the data processing apparatus does not malfunction), its frequency abnormality cannot be detected.